The state-of-the-art method for electrolyzing an alkali metal halide, especially sodium chloride (NaCl) or potassium chloride (KCl), is to use a fluorinated membrane to separate the anolyte and catholyte compartments of an electrolytic cell. The membrane permits the alkali metal cation to pass through to the catholyte, but severely restricts the undesirable passage of hydroxyl ion from the catholyte to the anolyte. To make membrane electrolysis attractive, the power consumption should be minimized, which means that the current efficiency should be maximized, and the resistance should be minimized.
Many efforts have been made to improve the performance of these membranes, particularly membranes used in chloralkali cells, by a wide variety of treatments. Most of the efforts have been aimed at obtaining lower voltage, higher current efficiency or lower power consumption.
When producing high concentrations of caustic, for example, 36% to 45% caustic, the chloralkali cell generally operates at a higher cell voltage and lower current efficiency than that of a cell producing lower concentrations of caustic. The elevated cell voltage is one factor that makes the electrolytic production of high concentrations of caustic cost prohibitive. Thus, one emphasis is on increasing current efficiency and lowering power consumption.
Variations in the voltage of the electrolyte cell have a direct affect on power consumption. Voltage cycles of as little as 30-50 mV can upset the balance of heat. If constant temperature is not maintained, the membrane will not operate under equilibrium conditions, and power consumption may be undesirably affected. Any voltage over that needed to electrolyze brine is lost as heat; the result is a waste of electric power. Similarly, excessive heat production can limit electrolyzer productivity by raising cell temperatures and increasing gas volume. Also, the rectifiers used in chloralkali plants are rated for power, which is the product of voltage and amperage. At higher voltages, less amperage can be supplied, thereby reducing the productivity of the electrolyzers.
Changes in cell voltage present other problems, as well. For example, small fluctuations in cell voltage can lead to an unsteady current density within the membrane which may lead to an undesirable increase in power consumption. Likewise, if the membrane is not operated under equilibrium conditions, water transport may be affected which could cause undesirable changes in concentrations of caustic.
It is known that the irradiation of a fluorinated cation exchange membrane may improve cell voltage. U.S. patent application Ser. No. 07/663,003 (allowed but not issued) filed by this inventor discloses and claims a process for irradiating a fluorinated cation exchange membrane. However, this patent does not teach or disclose a process for irradiating cation exchange membranes used to produce caustic at concentrations as high as 45% at high current efficiency and low power consumption.
As noted above, it is especially important to improve cell current efficiency and power consumption in electrolytic cells used to produce high concentrations of caustic.